Atmospheric corrosion-resistant coldrolled steel sheet of deep drawing quality



United States Patent ()filice 3,368,886 Patented Feb. 13, 1968 3,368,886 ATMOSPHERIC CORROSION-RESISTANT COLD- ROLLED STEEL SHEET F DEEP DRAWING QUALITY Toru Muta, Mineo Shimizu, Kameo Matsukura, and

Tadashi Nishi, Fukuoka Prefecture, Japan, assignors to Yawata Iron & Steel Co., Ltd., Tokyo, Japan No Drawing. Continuation-impart of application Ser. No. 238,523, Nov. 19, 1962. This application July 5, 1966, Ser. No. 562,474

Claims priority, application Japan, Dec. 11, 1961, 36/45,152 1 Claim. (Cl. 75125) ABSTRACT OF THE DISCLOSURE An atmospheric corrosion-resistant cold-rolled steel sheet is provided which is of deep drawing quality. The steel is composed of less than 0.01% C, less than 0.08% Si, 0.20 to 0.50% Mn, 0.05 to 0.12% P, 0.2 to 0.6 Cu, less than 0.05% As, less than 0.001% free N and the rest Fe accompanied by unavoidable impurities.

This application is a continuation-in-part of our copending application Ser. No. 238,523 filed Nov. 19, 1962, now abandoned.

This invention relates to an atmospheric corrosionresistant cold-rolled steel sheet of deep drawing quality.

Nowadays, low-carbon cold-rolled steel sheets of a rimmed steel or an aluminum-killed steel are generally used for steel sheets of deep drawing quality but such steel sheets are lacking in atmospheric corrosionresistance.

As pressed parts for automobile bodies, these steel sheets are undercoated and then painted to prevent corrosion after being assembled into the bodies. However, parts that are exposed to the ground, such as floor boards, fenders and rocker plates will rust earlier with great damage to the body. With the aim of reducing the production cost and weight of passenger cars, a tendency recently became popular of building frameless structures and retaining the strength and rigidity by means of the bodies, per se. However, there is a fear that any corrosion or break of the steel sheet of the body may cause a fatal defect in the strength of the body as compared with a structure having a frame. Therefore, for an underbody units of the automobile, galvanized steel sheets have been put to good use. Thegalvanized iron sheets show a high anticorrosiveness when used in an automobile; however, as Welding is mostly used in assembling bodies, the plating film around the welded part will be peeled ott by heat and will be also broken by splashed gravels and mechanical rubs. The sheet will begin to rust parts around the broken film and, further the deep-drawability of the sheet is not suificient. Thus the galvanized steel sheet can hardly be said to be perfectly satisfactory.

Today atmospheric corrosion-resistant high-tensile strength steels of various compositions have been used. They can be made thinner than ordinary steels and when so made the steel is required to be high-tensile strength. But, for obtaining atmospheric corrosion-resistance of high-tensile strength steels some elements would have to be added, which would, however, sacrifice the workability of the steel sheets to such an extent that, although, they may be endurable to light pressing works they would not be able to withstand hard drawing as required for steel sheets of deep drawing quality in automobiles.

An object of the present invention is to provide a steel sheet which is of the same tensile-strength as an ordinary steel sheet of deep-drawing quality but has both excellent resistance to atmospheric corrosion and deep-drawability.

The atmospheric corrosion-resistantsteel sheet having deep-drawability of the present invention is characterized by being composed of less than 0.01% C, less than 0.08% Si, 0.20 to 0.5% Mn, 0.05 to 0.12% P, 0.2 to 0.6% Cu, less than 0.05% As, less than 0.001% free N and the remainder being Fe and unavoidable impurities.

The steel sheet of the present invention can contain one or more of such elements having a high affinity for nitrogen, such as 0.01 to 0.08% Al, less than 0.1% V and less than 0.01% B depending on the circumstances.

In the present invention, carbon is contained in an amount of less than 0.01%, because it is preferable to be low for the benefit of deep-drawability and non-ageing properties, silicon is added for deoxidation in a range of less than 0.08% according to the steel making process. Manganese is to improve weldability as well as for deoxidation. However, when manganese is more than 0.5%, the tensile-strength will become higher and the workability will become lower. When it is less than 0.20%, the weldability and hot-rolling property will become low. Phosphorus increases the atmospheric corrosion-resistance and the tensile-strength, when it coexists with copper. But, when phosphorus is more than 0.12%, the steel will become harder. When it is less than 0.05%, the atmos pheric corrosion-resistance will be reduced. However, the most important effect of phosphorus is to improve, not only the atmospheric corrosion-resistance and tensilestrength, but also the deep-drawability. One of the most important features of the present invention is that phosphorus is contained in the steel of the present invention in an amount so as to be able to improve the deep-drawability of the steel. As is shown in the following examples, for instance, in the comparison of a steel sheet containing 0.017% phosphorus and that containing 0.065% phosphorus a distinct diiference can be perceived between them in the deep-drawability, as is demonstrated by conical cup value of 37.1 mm. for the former and 35.9 mm.- for the latter. This means that the steel sheet containing more phosphorus is superior in the deep-drawability.

Copper deteriorates the workability, when the content thereof exceeds 0.6%. Even if arsenic is not added, with phosphorus and copper, a practical resistance to atmospheric corrosion will be obtained. However, if the content of arsenic is less than 0.05%, the corrosion-resistance may improve without reducing the workability. Nitrogen (N not combined with other metals) in the steel sheet should be less than 0.001% in order that said steel sheet may be practically non-ageing.

In order to obtain a steel sheet having a composition according to the present invention and having superior resistance to atmospheric corrosion and also having a deep drawing quality, the chemical composition of the steel sheet must be adjusted in the steel making process and in the final heat-treating step after cold-rolling.

For this purpose, a molten steel composed of less than 0.10% C, less than 0.08% Si, 0.20 to 0.50% Mn, 0.05 to 0.12% P, 0.20 to 0.60% Cu, less than 0.05% As and the rest being Fe and unavoidable accompanying elements is poured into a mold and the thus obtained ingot is slabbed, hot-rolled, cold-rolled and annealed as in the normal way. In order to reduce oxygen in the steel which has a bad influence on the deep-drawability, the molten steel may be vacuum-degassed or may be cast in a vacuum or in an inert gas, or the vacuum-degassed molten steel may be further teemed in a vacuum or in an inert gas atmosphere. The thus treated steel has few non-metallic inclusions and therefore has a favorable surface property and is higher in deep-drawability.

After the teeming, the steel is treated in the same manner as an ordinary low-carbon steel sheet to be deep drawn. By cold-rolling at a reduction rate of 30 to 90%, a material coil of the right thickness for annealing will be obtained. This coil is annealed to obtain recrystallization while being positively decarburized and denitrided by using a proper atmospheric gas by means of a continuous annealing line or an open coil annealing furnace. If only decarburization is to be carried out, a mixture gas is used consisting of 3 to 20% hydrogen with the remainder being nitrogen (HNX gas), and if both decarburization and denitrification are to be carried out, a mixture gas is used consisting of more than 70% hydrogen with the remainder being nitrogen (AX gas). When the recrystallizing temperature has been reached, steam is introduced into the atmospheric gas in order to soak the material coil for the required time.

If it is desired to omit the denitriding heat-treatment mentioned above, one or more elements having a high atlinity for nitrogen, such as, for example, aluminum, vanadium and boron may be added to the starting molten steel in advance. In such case, 0.01 to 0.08% Al, less than 0.1% V and less than 0.01% B are added. However, these elements will have substantially no influence on the atmospheric corrosion-resistance and deep-drawability of the decarburized steel sheet.

Even when these elements are added, if the heat-treatment for denitrification is carried out, they will be more effective.

The thus obtained steel sheet contains less than 0.01% C, and less than 0.001% N (N not combined with other metals) when the denitrification treatment is carried out, thereby obtaining a steel having superior deep-drawability. Especially, by reducing nitrogen a practically sufiicient non-aging property is obtained and no deterioration occurs in the deepdrawability by strain ageing after skin pass rolling.

The characteristics of the steel sheet of the present invention shall be explained with reference to the follow ing examples.

Example 1 A cold-rolled coil 0.8 mm. thick was prepared by subjecting a molten steel having a ladle analysis shown in Table I to an ingot production process and thereafter subjecting the ingot to the required steps of slabbing, hotrolling, pickling and cold-rolling.

The cold-rolled coil was heated up to 740 C. in an HNX gas atmosphere in an open coil annealing furnace. Then the HNX gas atmosphere was changed to an AX gas atmosphere and a steam was introduced therein. The coil was further heated in the wet AX gas atmosphere of a dew point of 40 C. for 5 to hours to carry out a decarburization annealing.

When the carbon content of the product was reduced to less than 0.010% and free nitrogen to less than 0.001% by the decarburization annealing, the introduction of the steam was stopped and the wet atmosphere was again switched to the dry HNX gas, and the product was cooled. The product was subjected to a skin pass at a reduction rate of 0.8%.

The mechanical property, ageing property, deep-drawability and atmospheric corrosion-resistance of the products were as shown in Table II.

TABLE I.LADLE ANAPLgISIS OF EXAMPLE 1 (WEIGHT Bauhaus-Samples l3 and C are the products of the present lnven tion. Sample A is a normal steel, which is referred to for comparing with the products of the present invention.

TABLE II.PROPERTIES OF THE PRODUCTS OF THE PRESENT INVENTION Sample No.

A B C Tensile-strength in kg./mrn.'-' 30. 5 31. 0 30. 9 Yield point in kgJmrn. 15. 5 16.5 16. (J Elongation in percent (gauge length of 50 mm.). 46. 0 46. l 47. 1 Erichsen value in mm i. 11.0 11. 2 ll. 1 Conical cup value in mm. 37. 1 35. 8 35. 9 Strain ratio r 1. 44 1.68 1. 76 Ageing index in kgJrnm l. 2 1.0 0. 8 G.S. (A.S.'1.r\t.) 5. 6 8.1 7.8 Outdoor exposure test corrosion in gJdmfi/two years 1 Samples B and C show improvements not only in the atmospheric corrosion-resistance, but also in the deepdrawability as compared with those of Sample A. For instance, Samples B and C show lower values in conical cup value and much higher values in elongation, Erichsen value and strain ratio than Sample A. These values demonstrate that the products of the present invention are excellent in the press-forming property, that is, in the deep drawing property.

In the comparison of elements contained in the steels it is seen that Sample A and Samples B and C are different in the contents of phosphorus, copper and arsenic. As is already mentioned, all these three elements are effective on improving the atmospheric corrosion-resistance. However, as regards the deep-drawability, copper and arsenic do not specially contribute to the improvement of this property. Therefore, it was proved that the element which was effective on it was phosphorus.

Further, the content of free nitrogen in the products of the present invention was less than 0.001%.

Example 2 A cold-rolled coil 0.8 mm. thick made from a molten steel having a ladle composition of 0.08% C, 0.01% Si, 0.38% Mn, 0.09% P, 0.013% S, 0.41% Cu, 0.036 As and 0.0019% N and subjecting the ingot to the necessary steps from slabbing down to cold-rolling was heated to 700 C. in a dry hydrogen atmosphere of a dew point of 40 C., was then heated for 10 hours in a wet hydrogen atmosphere of a dew point of 30 C. by introducing steam, was cooled by again switching to the dry hydrogen atmosphere and was thus annealed for recrystallization. The chemical composition of the thus obtained product was 0.004% C, 0.01% Si, 0.38% Mn, 0.09% P, 0.012% S, 0.39% Cu, 0.0005% N, 0.036% As and impurities of 0.022% Ni, 0.017% Cr and 0.013% Sn. The mechanical properties of the product after the skin pass of a reduction rate of 0.6% were as follows:

After arti- Just after ilcial ageing skin pass at C. for 1 hour Tensile-strength in kg/mm. 31. 8 31. 0 Yield strength in kg./n1m. 22. 4 23. 0 Elongation in percent (gauge length of 50 min.) 45.1 44. 9 Erichsen value in mm. 10. 8 10. 6 Conical cup value in mm 35. 7 35. 7

6 than 0.001% free N and the rest being Fe and unavoid- 2,986,483 5/1961 Leslie et a1 75125 X able impurities. 3,177,070 4/1965 Wacquez 75-125 References Cited 3,193,417 7/1965 Kopchak 75123 X UNITED STATES PATENTS 3,215,567 11/1965 YOShida 75-423 X 2,109,271 2/ 1938 Krause 75-123 X DAVID L, RECK, Primary Examin r.

2,121,057 6/1938 Smith 75125 2,165,553 7/1939 Krause 75 125 P. WEINSTEIN, Asszstant Exammer. 

